Analog computer grinding control



p 18, 1967 s.w. DANIEL ETAL 3,314,6M

ANALOG COMPUTER GRINDING CONTROL Filed April 15, 1964 SEPARATOR 4/CLlNKER FEEDER 3 ELEVATOR 5 37 GYPSU (N Z5 M FEEDER Pnooucr l I 1 i9 IZ? Mu u. FEEDER C SPEED i ONTROLLER MICROPHONES .57

l 4 SOUND SENSOR -49 5.9

l sSouuo ENSOR 67 EEDER 6:9 I RATIO CONTROLLER 63 I \53 ELEVATOR IEmwggmc 4/ M MOTOR SYSTEM SET POINT Mo'ron INVENTORS STEWART W. DNIEL A7' TOR/VEV United States Patent Ofiice ifil ifil i Patented Apr. 18,1967 3,314,614 ANALUG CUMPUTER GRINDING CQNTROL Stewart W. Daniel and.lorgen U. Berni, Peterlrorongh, @ntario, Canada, assignors toMilltronics Limited, Peterborough, Ontario, Canada, a company of CanadaFiled Apr. 15, 196 5, Ser. No. 360,022 3 Claims. (Cl. 241-35) Thisinvention relates to apparatus and method for the control of grindingequipment such as a wet and/or dry ball mill, rod mill, tube mill,autogenous mill or the like, and more particularly to control apparatusand method which comprehends simultaneously developing severalcontinuous signals representative of variables involved in grindingprocess and combining these several signals in selectable amplitudeproportions in a computer-amplifier to produce control signals which canbe applied as required for stable high efiiciency operation to obtain aproduct having preselected characteristics at a preselectedsubstantially continuous production rate.

The mill grinding of raw materials to produce useful end products suchas cement and the like involves feeding one or more raw materials orores into a ball mill, rod mill, autogenous mill or the like apparatusand wet or dry grinding the materials fed in until they are broken upand intermixed to a product having preselected particle size and surfaceareas in a mixture of preselected constituent ratios. If all parametersof the arrangement and conditions within the grinding equipment remainconstant and stable, the quantity per unit time and quality of theproduct will also remain steady and stable. For short time periodsstable operation usually obtains in known grinding techniques. Forprotracted times of continuous operation, however, conditions in thegrinding circuit may vary with consequent undesirable effects on the endproduct. The grinding machinery is, of course, capable of performing afinite amount of work according to its design, said work comprising theovercoming of normal losses plus the grinding of the fed materials tothe required surface area of product and the removal of the product. Insome instances mill output is screened in a separator and oversizedparticles in the product are recirculated and refed into the mill forfurther grinding and reduction to the desired product size. if thediscrete particle size of a feed material increases, more work will beattributable to the recirculation chore and the quantity and/ or surfacearea of the end product will tend to decrease. Extraordinary increasesin feed particle size will quickly overload the grinding circuit.Conversely, a decrease in feed particle size will result in a lowercirculating load and a higher than specified product quality andproduction rate.

In the materials reduction art, known methods and ap paratus for controlof grinding mill equipment involve the adjustment of feed rates to themill of one or more raw materials in accordance with power consumptionof the grinding machinery drive motors and with mill noise. Powerconsumption is relative to the particle size and the hardness of thefeed material being milled and sounds emanating from the mill vary infrequency and/or amplitude according to the size, surface area andquantity of the particles in process. In systems where some portion offirst-milled product is recirculated to the mill inlet for furthergrinding, the power consumption in the recirculating system is relativeto recirculated volume and thus to the feed ratio of new material torecirculated material. Control of mill operation can be effected in anumber of ways to produce desired results. An operator of a cement mill,for example, may regulate for a constant surface area of product, withsome latitude tolerated in the production output rate, by decreasing andincreasing the feed rate. In addition, he may maintain the desiredsurface area of product at an intentionally lowered output rate byincreasing grinding time when the grindability or hardnesscharacteristics of the feed changes. With certain types of feedingcomponents operating at set speeds the feed rate varies with flowabilityof the feed, moisture content, line voltage variations in powersupplies, bin cavitations or hang-ups and feeder flushing. An increasein feed rate due to a change in one of such feedability variables willresult in an increased ore charge volume, increased circulating load andlarger discrete particle prod uct size with consequent reduction oftotal surface area per unit weight and volume in the end product.Conversely, a decreased feedability will tend to lead to mill grindoutand the end product may be overground to a higher than specified surfacearea. To control operation for these occurrences, mill loading may bemonitored and the feed rate controllably varied to maintain optimumgrinding circuit loading and/or the product output may be monitored, byweighing for example, and the feed rate varied accordingly.

As far is known, all of the above-described control techniques involve ahuman operator at some point in the arrangement. Persons skilled in theart recognize that control dependent on a hum-an operator is prone tosubjective error, inaccuracy, insensitivity and unsatisfactoryreproducibility. While many modern control systems utilizesignal-producing monitoring devices such as indicating and recordingwattmeters, microphones with sound amplifiers or visual indicators andthe like, no completely automated and completely satisfactory system forgrinding mill control has been available to industry prior to the timeof the present invention.

It is, then, the primary object of the present invention to provide afully automated apparatus arrangement and method for controllinggrinding mills and equivalent equipment which, in normal operation, iscompletely free from subjective error attending systems involving humanoperators.

it is a further object of this invention to provide grindi g millcontrol wherein signals developed through the monitoring of severaldifferent process variables are combined to produce useful continuouscontrol signals which are applied to effect changes in desireddirections of the monitored variables.

It is a still further object of the present invention to provide forcombining several signals produced by monitoring process variables inselectable ratios of a signal valve or amplitude.

A still further object of the present invention is to provide novelelectronic circuitry for inverting signal amplitudes to render themsuitable for combination with other signals in the overall controlscheme.

Another important object of the invention is to furnish a control systemcapable of initiating corrective and alarm actions upon the occurrenceof malfunctions and extraordinary conditions.

With the foregoing and other objects in view which will become moreapparent hereinafter, the invention will now be described with greaterparticularity and with reference to the drawing which is a schematicdiagram of a control system according to our invention.

In general, the present invention comprehends monitoring separatevariables in a grinding process with components capable of producingdiscrete signals representative of instantaneous values of saidvariables, adjusting the respective amplitudes and phases of signals sodeveloped, combining the several signals additively con tinuously in acomputer and developing in said computer a control signal which can beapplied to efiect changes in at least one operating parameter of thegrinding system.

In the drawings, there is shown a two-compartment cement grinding mill11 having an inlet or charging end 13 and an outlet or discharge end 15.A charging header .ro phone.

17 connects to the inlet 13 and manifolds to a gypsum feeder 19 and aclinker feeder 21. The mill outlet connects to a discharge header 23which in turn connects to an elevator 25. Elevator 25 empties outputfrom the mill into a conveyor 27 which carries the particlized output toa separator 29. In separator 29, the mill output is screenedcontinuously and separated into product particles having the desiredsurface area characteristics and tailings or particles of larger thanthe preselected size. Product particles are continuously removed througha product header 31 and bagged, stored or otherwise handled. Thetailings are converged through a recirculating tailings header 33 to thecharging header 17, adding the tailings to the continuously suppliedcharges from the gypsum feeder 19 and the clinker feeder 21.

A clinker feeder speed controller 35 is operably connected to clinkerfeeder 21 and a gypsum feeder speed controller is similarly connected tothe gypsum feeder 19. A feeder ratio controller 39 connects to the speedcontrollers and provides for adjustment of the feed ratio. Elevatormotor 41 provides driving power for the elevator 2S and separator motor43 provides driving power for the separator 29.

In any particular control system embodying the present invention, thefirst decision to be made is to select the type, location and number ofsensing or monitoring devices. Persons skilled in the art will recognizethe importance of this consideration and will also recognize thatconventional methods of system analysis will readily provide exactdesign answers in applying the present invention. The type, location andnumber of system sensing points is governed by the complexity of thegrinding circuit, the system time constants, the duty limitations ofdiscrete components in the grinding circuit and the economics of theparticular grinding operation. A grinding arrangement involving twocascaded mills with two elevators and separators may require three ormore sensors. A single compartment ball mill in a simple grinding systemmay be made to perform quite acceptably with one sensor, probably asound sensor. High powered multicompartment mills may operate optimallywith a sound sensor for each separate compartment. The typical twocompartment closed circuit system can usually be satisfied with onesound sensor and one power sensor. However, if the second compartmenttends to overload for any reason, addition of a second sound sensor maybe indicated. For the purposes of this description a two compartmentmill, elevator, separator, recirculation and monitoring with two soundsensors and two power sensors will be considered.

' Three types of sensors useful in connection with the practice of thisinvention are presently commercially available scil., sound sensors,power sensors and tem perature sensors. The sound sensor is essentiallya mic- Sound emanating from a grinding mill consists basically of noisegenerated by steep wave-front impact sounds and contains frequenciesfrom below 100 c.p.s. to well over 100,000 c.p.s. Ambient sounds frommotors, gear boxes, bearings and other machine components arepredominantly in the 60 c.p.s. to 1500 c.p.s. portion of the spectrum,with peaks occurring at 60, 120 and 180 cycles, corresponding to theline frequency and its secondfand third harmonics. Sound sensors used inmonitoring chores according to this invention are customarily providedwith suitable band pass filters to reject unwanted frequencies andensure the development of control signals representative of grindingcircuit performance. Commercially available electronic watt convertersare suitable for use as power sensors. The power consumed by an electricdrive motor is directly related to the amount of work being performed.In connection with elevators, conveyors, separators and the likeequipment in grinding systems, the power sensor provides for monitoringof grinding circuit throughout and/ or circulating load. Temperaturesensors in the form of resistance temperature detectors may also beinstalled at points in the system where temperature variations are to bemonitored towards determining work load. As noted hereinabove, thepresent invention will be described by considering two microphones andtwo power sensors.

Referring to the drawing, the mill compartment adjacent inlet 13 has afirst microphone 45 arranged in juxtaposition thereto. Similarly, asecond microphone 47 is installed adjacent the second compartment of themill 11. These microphones 45, 47 connect respectively to electronicsound sensors and inverters 49 and 51. The power circuit to elevatormotor 41 is monitored by a first electronic watt converter 53 and theseparator drive motor 43 is monitored by another electronic wattconverter 55.

The circuits shown schematically at 5'7, 59, 61, and 63 conduct therespective signals developed in sound sensors 49, 51 and electronic wattconverters 53, to input points on a computing amplifier 65. In computingamplifier 65, the several continuously received signals from the sensorsor monitoring devices are combined according to preselected proportionsand made to develop a feeder control signal which is continuouslyapplied to the feeder speed controllers 35, 37 through circuit 6 and thefeeder ratio controller 39. In other words, the output terminals of thecomputing amplifier deliver a voltage or current suitable forcontrolling the rate of feed and the feed ratio for the grindingcircuit. Conn puting amplifier is provided with a manually adjustablesystem set point potentiometer 71. The computer amplifier operates sothat the sum of all sensor signals is revealed to any preselected setpoint on potentiometer 71 and a tendency of one or more sensors todepart from the set point above or below preselected limits will cause acorrective or alarm signal to be initiated. In normal operation thepotentiometer '71 is readjusted only when changing productspecifications. When the output voltage of the computing amplifierexceeds the voltage re quired to produce a maximum feed rate, thefeeders are not supplying sufficient feed volume to satisfy the systemset points. Timing circuitry in the computer amplifier provides for theinitiation of a feed deficiency alarm signal or corrective signal whenthe excess voltage obtains for a preselected period.

The monitoring signals from the microphones and electronic wattconverters are produced and utilized as follows: Microphone 4:7 detectsin accordance with sound impinging thereon changes in feed size,grindability and feed rate. Since in time and flow sequence microphone45 is ahead of microphone 47, it will lead microphone 47 and both of thepower sensors or electronic watt converters 53, 55 in signal productionby some finite time. Microphone 47 produces a signal indicative of thecorn dition of the feed charge in the second compartment of mill 11. Theelectronic watt converter 53 produces a, signal representative of thepower required to elevate the product plus the circulating load. Theadditive effect of the signal produced by converter 53 can be adjustedaccording to circulating load volume. Electronic watt converter 55produces a signal representative of power consumed by the separatormotor drive. This signal can also be adjusted for varying additiveeffect in development of the cumulative control signal produced in thecomputing amplifier 65.

Separate sensor ratio summing controllers in computing amplifier 65permit selection of the desired proportional bands required to producethe desired amount of corrective action from each sensor. Each componentof the grinding circuit, first compartment, second compartment, elevatorand separator has its own particular design overload point. Underabnormal or transient grinding conditions one or more of thesecomponents could be overloaded even though other components of thecircuit were operating satisfactorily. This possibility requires thateach sensor be individually capable of reducing the feed rate if andwhen its signal amplitude exceeds a preset value representing overloadlimit. Below this point, the sensor signal will contribute its voltageto the summing network in computing amplifier 65 as a controllingsignal. Above the pre-established value said sensor will effect timeoverriding action a reduction in the feed rate irrespective of thesignal values from the other sensors. The computing amplifiers circuitryalso includes a minimum feed rate adjustment, a maximum feed rateadjustment and a reset action, particularly useful in coping with thewide time lags encountered in grinding operation.

In a particular embodiment of the present invention, the computingamplifier used is a Model CAIO computing amplifier manufactured byMilltronics, Ltd., Peterborough, Canada. The electronic watt convertersemployed are Model WA11 Watt meter amplifiers made by Milltronics, supraand the sound sensors Model $813 of the same manufacturer.

The equipment components used and arranged in the combination accordingto the present invention can be seen then to provide an extraordinaryflexible and precise control method and apparatus arrangement. While theessential rationale of the invention involves the continuous monitoringof several separate points in the grinding circuit and the additivecombination of signal indicia representative of variables of interest inthe process and combination of these to produce a useful control signal,many additional benefits are provided such as malfunction alarms andprovision for overriding control signals upon the occurrence ofextraordinary conditions in the process.

The present invention therefore can be seen to constitute an unusuallysophisticated advance in the art. While, for the purposes of describingthe present invention a specific embodiment has been treated in detailsufiicient to permit practice thereof, there will undoubtedly occur topersons skilled in this art many modifications which will come withinthe spirit of our invention. Accordingly, this disclosure should beconstrued as illustrative of our invention and in no way in a limitingsense, the scope of the invention being defined by the appended claims.

What is claimed is:

1. A method for grinding mill control which method comprises monitoringat least one sound variable incident to the grinding process, whichsound variable is representative of a particular condition of materialin process; developing a first signal representative of theinstantaneous value of the sound variable monitored; monitoring at leastone power consumption variable, which power consumption variable isrepresentative of a condition of the material in process; developing asecond signal representative of the instantaneous value of the powervariable monitored; combining said first signal and said second signalin a computer; developing in said computer a control signal proportionalto and representative of the cornbined first and second signals andapplying said control signal to effect changes in a process parameterwhich bears on the values of the signals monitored.

2. A method for grinding mill control which method comprisescontinuously monitoring sound emanating from a grinding mill; developinga first signal having amplitude proportional to and representative ofthe amplitude of the sound monitored; continuously monitoring powerconsumption of at least one driving element in the grinding millcircuit; developing a second signal having an amplitude proportional toand representative of the instantaneous value of power being monitored;adjusting the amplitude of at least one of said signals to effect apreselected amplitude ratio of said first signal to said second signal;combining said first signal and said second signal in a computer;developing a control signal having an amplitude representative of thecombined value of the first signal amplitude and the second signalamplitude and applying said control signal to feeder speed controlapparatus to effect changes in feed rate of raw materials being fed tothe grinding mill.

3. Apparatus for the control of a grinding mill used to effect reductionin particle size of raw materials fed thereto continuously, whichapparatus comprises in combination sound detector means disposed tocontinuously monitor sound emanating from said mill; means forconverting said sound monitored to a first continuous signal having anamplitude representative of the instantaneous amplitude of the soundmonitored; electric power measuring means disposed to continuouslymeasure power consumed by at least one electric monitor drive element inthe grinding mill circuit; means for developing a second signal havingan amplitude representative of the power measured by said last-namedmeans; computer means in circuit with said means for developing saidfirst sign-a1 and means for developing said second signal, said computermeans being adapted to combine said first signal and said second signalinto a control signal and to apply said control signal to effect changesin the mill feed characteristics.

References Cited by the Examiner UNITED STATES PATENTS 2,766,939 10/1956Weston 24l34 X 2,833,482 5/1958 Weston et a1 241-34 X 3,094,289 6/1963Fahlstrom 24134 3,145,935 8/1964 Wilson 241-34 X WILLIAM W. DYER, JR.,Primary Examiner. H. F. PEPPER, Assistant Examiner.

1. A METHOD FOR GRINDING MILL CONTROL WHICH METHOD COMPRISES MONITORINGAT LEAST ONE SOUND VARIABLE INCIDENT TO THE GRINDING PROCESS, WHICHSOUND VARIABLE IS REPRESENTATIVE OF A PARTICULAR CONDITION OF MATERIALIN PROCESS; DEVELOPING A FIRST SIGNAL REPRESENTATIVE OF THEINSTANTANEOUS VALUE OF THE SOUND VARIABLE MONITORED; MONITORING AT LEASTONE POWER CONSUMPTION VARIABLE, WHICH POWER CONSUMPTION VARIABLE ISREPRESENTATIVE OF A CONDITION OF THE MATERIAL IN PROCESS; DEVELOPING ASECOND SIGNAL REPRESENTATIVE OF THE INSTANTANEOUS VALUE OF THE POWERVARIABLE MONITORED; COMBINING SAID FIRST SIGNAL AND SAID SECOND SIGNALIN A COMPUTER; DEVELOPING IN SAID COMPUTER A CONTROL SIGNAL PROPORTIONALTO AND REPRESENTATIVE OF THE COMBINED FIRST AND SECOND SIGNALS ANDAPPLYING SAID CONTROL SIGNAL TO EFFECT CHANGES IN A PROCESS PARAMETERWHICH BEARS ON THE VALUES OF THE SIGNALS MONITORED.